Re: [PATCH] x86: remove arbitrary instruction size limit in instruction decoder

[Masami Hiramatsu]

(2014/11/13 7:53), Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> 
> The current x86 instruction decoder steps along through the
> instruction stream but always ensures that it never steps farther
> than the largest possible instruction size (MAX_INSN_SIZE).
> 
> The MPX code is now going to be doing some decoding of userspace
> instructions.  We copy those from userspace in to the kernel and
> they're obviously completely untrusted coming from userspace.  In
> addition to the constraint that instructions can only be so long,
> we also have to be aware of how long the buffer is that came in
> from userspace.  This _looks_ to be similar to what the perf and
> kprobes is doing, but it's unclear to me whether they are
> affected.
> 
> We shouldn't simply error out when we get short copy_from_user*()
> results from userspace (like intel_pmu_pebs_fixup_ip() does
> currently).  It is perfectly valid to be executing an instruction
> within MAX_INSN_SIZE bytes of an unreadable page. We should be
> able to gracefully handle short reads in those cases.
> 
> This adds support to the decoder to record how long the buffer
> being decoded is and to refuse to "validate" the instruction if
> we would have gone over the end of the buffer to decode it.
> 
> The kprobes code probably needs to be looked at here a bit more
> carefully.  This patch still respects the MAX_INSN_SIZE limit
> there but the kprobes code does look like it might be able to
> be a bit more strict than it currently is.

Would you mean kprobes can copy shorter? Maybe, but I think current
one is enough because it is on a cold path.
OK, at least this looks good to me.

> 
> Note: the v10 version of the MPX patches I just posted depends
> on this patch.

BTW, current insn decoder doesn't support MPX... That should be
updated (add bnd* to x86-insn-map.txt)


Acked-by: Masami Hiramatsu <masmai.hiramatsu.pt@xxxxxxxxxxx>

> 
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Jim Keniston <jkenisto@xxxxxxxxxx>
> Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
> Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@xxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> ---
> 
>  b/arch/x86/include/asm/insn.h                |   10 ++++++----
>  b/arch/x86/kernel/cpu/perf_event_intel_ds.c  |    9 ++++++---
>  b/arch/x86/kernel/cpu/perf_event_intel_lbr.c |    2 +-
>  b/arch/x86/kernel/kprobes/core.c             |    8 +++++---
>  b/arch/x86/kernel/kprobes/opt.c              |    4 +++-
>  b/arch/x86/kernel/uprobes.c                  |    2 +-
>  b/arch/x86/lib/insn.c                        |    5 +++--
>  b/arch/x86/tools/insn_sanity.c               |    2 +-
>  b/arch/x86/tools/test_get_len.c              |    2 +-
>  9 files changed, 27 insertions(+), 17 deletions(-)
> 
> diff -puN arch/x86/include/asm/insn.h~x86-insn-decoder-remove-arbitrary-limit arch/x86/include/asm/insn.h
> --- a/arch/x86/include/asm/insn.h~x86-insn-decoder-remove-arbitrary-limit	2014-11-12 12:45:52.952753062 -0800
> +++ b/arch/x86/include/asm/insn.h	2014-11-12 12:45:52.969753829 -0800
> @@ -65,6 +65,7 @@ struct insn {
>  	unsigned char x86_64;
>  
>  	const insn_byte_t *kaddr;	/* kernel address of insn to analyze */
> +	const insn_byte_t *end_kaddr;	/* kernel address of last insn in buffer */
>  	const insn_byte_t *next_byte;
>  };
>  
> @@ -96,7 +97,7 @@ struct insn {
>  #define X86_VEX_P(vex)	((vex) & 0x03)		/* VEX3 Byte2, VEX2 Byte1 */
>  #define X86_VEX_M_MAX	0x1f			/* VEX3.M Maximum value */
>  
> -extern void insn_init(struct insn *insn, const void *kaddr, int x86_64);
> +extern void insn_init(struct insn *insn, const void *kaddr, int buf_len, int x86_64);
>  extern void insn_get_prefixes(struct insn *insn);
>  extern void insn_get_opcode(struct insn *insn);
>  extern void insn_get_modrm(struct insn *insn);
> @@ -115,12 +116,13 @@ static inline void insn_get_attribute(st
>  extern int insn_rip_relative(struct insn *insn);
>  
>  /* Init insn for kernel text */
> -static inline void kernel_insn_init(struct insn *insn, const void *kaddr)
> +static inline void kernel_insn_init(struct insn *insn,
> +				    const void *kaddr, int buf_len)
>  {
>  #ifdef CONFIG_X86_64
> -	insn_init(insn, kaddr, 1);
> +	insn_init(insn, kaddr, buf_len, 1);
>  #else /* CONFIG_X86_32 */
> -	insn_init(insn, kaddr, 0);
> +	insn_init(insn, kaddr, buf_len, 0);
>  #endif
>  }
>  
> diff -puN arch/x86/kernel/cpu/perf_event_intel_ds.c~x86-insn-decoder-remove-arbitrary-limit arch/x86/kernel/cpu/perf_event_intel_ds.c
> --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c~x86-insn-decoder-remove-arbitrary-limit	2014-11-12 12:45:52.954753152 -0800
> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c	2014-11-12 12:45:52.970753874 -0800
> @@ -724,6 +724,7 @@ static int intel_pmu_pebs_fixup_ip(struc
>  	unsigned long ip = regs->ip;
>  	int is_64bit = 0;
>  	void *kaddr;
> +	int size;
>  
>  	/*
>  	 * We don't need to fixup if the PEBS assist is fault like
> @@ -758,11 +759,12 @@ static int intel_pmu_pebs_fixup_ip(struc
>  		return 1;
>  	}
>  
> +	size = ip - to;
>  	if (!kernel_ip(ip)) {
> -		int size, bytes;
> +		int bytes;
>  		u8 *buf = this_cpu_read(insn_buffer);
>  
> -		size = ip - to; /* Must fit our buffer, see above */
> +		/* 'size' must fit our buffer, see above */
>  		bytes = copy_from_user_nmi(buf, (void __user *)to, size);
>  		if (bytes != 0)
>  			return 0;
> @@ -780,11 +782,12 @@ static int intel_pmu_pebs_fixup_ip(struc
>  #ifdef CONFIG_X86_64
>  		is_64bit = kernel_ip(to) || !test_thread_flag(TIF_IA32);
>  #endif
> -		insn_init(&insn, kaddr, is_64bit);
> +		insn_init(&insn, kaddr, size, is_64bit);
>  		insn_get_length(&insn);
>  
>  		to += insn.length;
>  		kaddr += insn.length;
> +		size -= insn.length;
>  	} while (to < ip);
>  
>  	if (to == ip) {
> diff -puN arch/x86/kernel/cpu/perf_event_intel_lbr.c~x86-insn-decoder-remove-arbitrary-limit arch/x86/kernel/cpu/perf_event_intel_lbr.c
> --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c~x86-insn-decoder-remove-arbitrary-limit	2014-11-12 12:45:52.956753243 -0800
> +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c	2014-11-12 12:45:52.970753874 -0800
> @@ -518,7 +518,7 @@ static int branch_type(unsigned long fro
>  #ifdef CONFIG_X86_64
>  	is64 = kernel_ip((unsigned long)addr) || !test_thread_flag(TIF_IA32);
>  #endif
> -	insn_init(&insn, addr, is64);
> +	insn_init(&insn, addr, size, is64);
>  	insn_get_opcode(&insn);
>  
>  	switch (insn.opcode.bytes[0]) {
> diff -puN arch/x86/kernel/kprobes/core.c~x86-insn-decoder-remove-arbitrary-limit arch/x86/kernel/kprobes/core.c
> --- a/arch/x86/kernel/kprobes/core.c~x86-insn-decoder-remove-arbitrary-limit	2014-11-12 12:45:52.957753288 -0800
> +++ b/arch/x86/kernel/kprobes/core.c	2014-11-12 12:45:52.970753874 -0800
> @@ -285,7 +285,7 @@ static int can_probe(unsigned long paddr
>  		 * normally used, we just go through if there is no kprobe.
>  		 */
>  		__addr = recover_probed_instruction(buf, addr);
> -		kernel_insn_init(&insn, (void *)__addr);
> +		kernel_insn_init(&insn, (void *)__addr, MAX_INSN_SIZE);
>  		insn_get_length(&insn);
>  
>  		/*
> @@ -330,8 +330,10 @@ int __copy_instruction(u8 *dest, u8 *src
>  {
>  	struct insn insn;
>  	kprobe_opcode_t buf[MAX_INSN_SIZE];
> +	unsigned long recovered_insn =
> +		recover_probed_instruction(buf, (unsigned long)src);
>  
> -	kernel_insn_init(&insn, (void *)recover_probed_instruction(buf, (unsigned long)src));
> +	kernel_insn_init(&insn, (void *)recovered_insn, MAX_INSN_SIZE);
>  	insn_get_length(&insn);
>  	/* Another subsystem puts a breakpoint, failed to recover */
>  	if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION)
> @@ -342,7 +344,7 @@ int __copy_instruction(u8 *dest, u8 *src
>  	if (insn_rip_relative(&insn)) {
>  		s64 newdisp;
>  		u8 *disp;
> -		kernel_insn_init(&insn, dest);
> +		kernel_insn_init(&insn, dest, insn.length);
>  		insn_get_displacement(&insn);
>  		/*
>  		 * The copied instruction uses the %rip-relative addressing
> diff -puN arch/x86/kernel/kprobes/opt.c~x86-insn-decoder-remove-arbitrary-limit arch/x86/kernel/kprobes/opt.c
> --- a/arch/x86/kernel/kprobes/opt.c~x86-insn-decoder-remove-arbitrary-limit	2014-11-12 12:45:52.959753378 -0800
> +++ b/arch/x86/kernel/kprobes/opt.c	2014-11-12 12:45:52.971753919 -0800
> @@ -251,13 +251,15 @@ static int can_optimize(unsigned long pa
>  	/* Decode instructions */
>  	addr = paddr - offset;
>  	while (addr < paddr - offset + size) { /* Decode until function end */
> +		unsigned long recovered_insn;
>  		if (search_exception_tables(addr))
>  			/*
>  			 * Since some fixup code will jumps into this function,
>  			 * we can't optimize kprobe in this function.
>  			 */
>  			return 0;
> -		kernel_insn_init(&insn, (void *)recover_probed_instruction(buf, addr));
> +		recovered_insn = recover_probed_instruction(buf, addr);
> +		kernel_insn_init(&insn, (void *)recovered_insn, MAX_INSN_SIZE);
>  		insn_get_length(&insn);
>  		/* Another subsystem puts a breakpoint */
>  		if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION)
> diff -puN arch/x86/kernel/uprobes.c~x86-insn-decoder-remove-arbitrary-limit arch/x86/kernel/uprobes.c
> --- a/arch/x86/kernel/uprobes.c~x86-insn-decoder-remove-arbitrary-limit	2014-11-12 12:45:52.961753468 -0800
> +++ b/arch/x86/kernel/uprobes.c	2014-11-12 12:45:52.971753919 -0800
> @@ -219,7 +219,7 @@ static int uprobe_init_insn(struct arch_
>  {
>  	u32 volatile *good_insns;
>  
> -	insn_init(insn, auprobe->insn, x86_64);
> +	insn_init(insn, auprobe->insn, sizeof(auprobe->insn), x86_64);
>  	/* has the side-effect of processing the entire instruction */
>  	insn_get_length(insn);
>  	if (WARN_ON_ONCE(!insn_complete(insn)))
> diff -puN arch/x86/lib/insn.c~x86-insn-decoder-remove-arbitrary-limit arch/x86/lib/insn.c
> --- a/arch/x86/lib/insn.c~x86-insn-decoder-remove-arbitrary-limit	2014-11-12 12:45:52.962753513 -0800
> +++ b/arch/x86/lib/insn.c	2014-11-12 12:45:52.972753964 -0800
> @@ -28,7 +28,7 @@
>  
>  /* Verify next sizeof(t) bytes can be on the same instruction */
>  #define validate_next(t, insn, n)	\
> -	((insn)->next_byte + sizeof(t) + n - (insn)->kaddr <= MAX_INSN_SIZE)
> +	((insn)->next_byte + sizeof(t) + n < (insn)->end_kaddr)
>  
>  #define __get_next(t, insn)	\
>  	({ t r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
> @@ -50,10 +50,11 @@
>   * @kaddr:	address (in kernel memory) of instruction (or copy thereof)
>   * @x86_64:	!0 for 64-bit kernel or 64-bit app
>   */
> -void insn_init(struct insn *insn, const void *kaddr, int x86_64)
> +void insn_init(struct insn *insn, const void *kaddr, int buf_len, int x86_64)
>  {
>  	memset(insn, 0, sizeof(*insn));
>  	insn->kaddr = kaddr;
> +	insn->end_kaddr = kaddr + buf_len;
>  	insn->next_byte = kaddr;
>  	insn->x86_64 = x86_64 ? 1 : 0;
>  	insn->opnd_bytes = 4;
> diff -puN arch/x86/tools/insn_sanity.c~x86-insn-decoder-remove-arbitrary-limit arch/x86/tools/insn_sanity.c
> --- a/arch/x86/tools/insn_sanity.c~x86-insn-decoder-remove-arbitrary-limit	2014-11-12 12:45:52.964753604 -0800
> +++ b/arch/x86/tools/insn_sanity.c	2014-11-12 12:45:52.972753964 -0800
> @@ -254,7 +254,7 @@ int main(int argc, char **argv)
>  			continue;
>  
>  		/* Decode an instruction */
> -		insn_init(&insn, insn_buf, x86_64);
> +		insn_init(&insn, insn_buf, sizeof(insn_buf), x86_64);
>  		insn_get_length(&insn);
>  
>  		if (insn.next_byte <= insn.kaddr ||
> diff -puN arch/x86/tools/test_get_len.c~x86-insn-decoder-remove-arbitrary-limit arch/x86/tools/test_get_len.c
> --- a/arch/x86/tools/test_get_len.c~x86-insn-decoder-remove-arbitrary-limit	2014-11-12 12:45:52.966753694 -0800
> +++ b/arch/x86/tools/test_get_len.c	2014-11-12 12:45:52.972753964 -0800
> @@ -149,7 +149,7 @@ int main(int argc, char **argv)
>  				break;
>  		}
>  		/* Decode an instruction */
> -		insn_init(&insn, insn_buf, x86_64);
> +		insn_init(&insn, insn_buf, sizeof(insn_buf), x86_64);
>  		insn_get_length(&insn);
>  		if (insn.length != nb) {
>  			warnings++;
> _
> 
> 


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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